The present invention relates to the fabrication of integrated circuits by chemical vapor deposition in a vacuum chamber. More particularly, the invention relates to a method and apparatus that enable the formation of high quality CVD films using both low temperature (e.g., about 400xc2x0 C.) and high temperature (e.g., above about 580xc2x0 C.) processing. The present invention is particularly useful in the deposition of TEOS-based (tetraethylorthosilicate) and silane-based chemistries including PECVD and SACVD deposition of silicon oxide, silicon nitride, silicon oxynitride and amorphous silicon as well as doped silicon oxides such as boron phosphorus silicate glass, phosphorus silicate glass and fluorine-doped silicate glass. The present invention may also, however, be used with other deposition chemistries.
One of the primary steps in the fabrication of modern semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of gases. Such a deposition process is referred to as chemical vapor deposition or CVD. Conventional thermal CVD processes supply reactive gases to the substrate surface where heat-induced chemical reactions take place to produce a desired film.
An alternative method of depositing layers over a substrate includes plasma enhanced CVD (PECVD) techniques. Plasma enhanced CVD techniques promote excitation and/or dissociation of the reactant gases by the application of radio frequency (RF) energy to a reaction zone near the substrate surface, thereby creating a plasma. The high reactivity of the species in the plasma reduces the energy required for a chemical reaction to take place, and thus lowers the temperature required for such CVD processes as compared to conventional thermal CVD processes. The relatively low temperature of some PECVD processes helps semiconductor manufacturers lower the overall thermal budget in the fabrication of some integrated circuits.
Semiconductor device geometries have dramatically decreased in size since such devices were first introduced several decades ago. Since then, integrated circuits have generally followed the eighteen month/half-size rule (often called xe2x80x9cMoore""s Lawxe2x80x9d), which means that the number of devices that will fit on a chip quadruples every eighteen months. Today""s wafer fabrication plants are routinely producing integrated circuits having 0.5-xcexcm and even 0.25-xcexcm features, and tomorrow""s plants soon will be producing devices having even smaller geometries.
Such decreases in size have been made possible in part by advances in technology associated with semiconductor manufacturing equipment, such as the substrate processing chambers used for PECVD processing. Some of the technology advances include advances that are reflected in the design and manufacture of certain CVD deposition systems in use in fabrication facilities today, while others are in various stages of development and will soon be in widespread use throughout the fabrication facilities of tomorrow.
One technology advance commonly used in today""s fabrication facilities includes the use of a PECVD technique often referred to as mixed frequency PECVD in which both high and low frequency RF power are employed to generate a plasma and to promote ion bombardment of a substrate. One such mixed frequency method couples both high and low frequency RF power to a metal gas distribution manifold that acts as a first electrode. In this method, application of the high frequency RF power is the primary mechanism that dissociates the reactant gases while application of the low frequency RF power promotes ion bombardment of a substrate positioned on a grounded substrate support that also functions as a second electrode. Another mixed frequency method couples high frequency RF power to a gas distribution manifold (first electrode) and couples low frequency RF power to a substrate holder (second electrode).
Another technology advance used in some currently available PECVD deposition chambers includes the use of conical holes in the gas distribution manifold to increase the dissociation of gases introduced into a chamber. A more detailed description of such conical holes is contained in U.S. Pat. No. 4,854,263, entitled xe2x80x9cINLET MANIFOLD AND METHODS FOR INCREASING GAS DISSOCIATION AND FOR PECVD OF DIELECTRIC FILMS,xe2x80x9d and having Mei Chang, David Wang, John White and Dan Maydan listed as co-inventors. The ""263 patent is assigned to Applied Materials, the assignee of the present patent application, and is hereby incorporated by reference in its entirety.
An example of a technology advance that is more recent than those noted above is the use of ceramics in a CVD chamber to allow the reactor to be used in high temperature operations. One CVD chamber that is specifically designed for such high temperature processing and includes a ceramic heater assembly among other features of the chamber is described in the 08/800,896 application noted above.
Advances in technology such as those just described are not without restrictions. For example, while mixed frequency PECVD techniques have proved to be very beneficial in a variety of applications, the simultaneous application of the high and low frequency waveforms must be controlled to avoid interferences which can result in high voltages and arcing at the gas distribution manifold. Arcing may be evidenced by a glow within the holes in the gas distribution manifold, and by a reduction in deposition rate as the amplitude of the high frequency voltage is increased. Arcing is typically avoided using one or more of the following techniques: maintaining the pressure within the vacuum chamber above a de minimis level for a particular process, operating with the low frequency RF power set at a value less than 30% of the total RF power, and/or reducing total RF power.
In the past, experiments had been performed in which conical holes were employed in a mixed frequency PECVD chamber having both the high and low frequency RF power sources connected to the gas distribution manifold. In these experiments, it was found that the arcing problem was further increased to the point that it substantially interfered with film deposition. Thus, all mixed frequency PECVD systems known to the inventors use straight, rather than conical, holes in the gas distribution manifold.
Accordingly, it is desirable to develop technology for substrate deposition chambers that enables semiconductor manufacturers to simultaneously take advantage of conical holes and mixed frequency PECVD deposition techniques.
The present invention provides an improved method and apparatus for depositing CVD films on a substrate. The apparatus employs mixed frequency RF power and includes a gas distribution manifold with conical holes. The potential for arcing is greatly reduced by connecting the low frequency RF power source to an electrode embedded in the substrate holder and connecting the high frequency RF power source to the gas distribution manifold, which also functions as an electrode. An independent matching network decouples the low frequency waveform from the high frequency waveform to minimize phase interferences between the waveforms.
These features combine to allow deposition processes to proceed at conditions that were unattainable in prior substrate processing chambers and also enable the substrate processing apparatus of the present invention to be usable in sub-0.35 xcexcm deposition processes including 0.25 and 0.18 xcexcm processes.
A substrate processing system according to one embodiment of the present invention includes a ceramic substrate holder with an embedded RF electrode and a gas distribution manifold spaced apart from the substrate holder. The gas distribution manifold supplies one or more process gases through multiple conical holes to a reaction zone of a substrate processing chamber within the processing system and also acts as a second RF electrode. Each conical hole has an outlet that opens into the reaction zone and an inlet spaced apart from the outlet that is smaller in diameter than the outlet. A mixed frequency RF power supply is connected to the substrate processing system with a high frequency RF power source connected to the gas inlet manifold electrode and a low frequency RF power source connected to the substrate holder electrode. An RF filter and matching network decouples the high frequency waveform from the low frequency waveform. Such a configuration allows for an enlarged process regime and provides for deposition of films, including silicon nitride films, having physical characteristics that were previously unattainable.
In one preferred embodiment of the method of the present invention, a silicon nitride film is deposited. A process gas including silane, ammonia and molecular nitrogen is introduced through a gas distribution manifold having conical holes and a plasma is formed from the process gas using mixed frequency RF power. The high frequency (HF) component is applied to the gas distribution manifold while the low frequency (LF) component is applied to a bottom electrode. It has been demonstrated that silicon nitride films deposited according to this embodiment under low temperature processing conditions can have a wet etch rate (WER) as low as 170 xc3x85/min while retaining excellent step coverage properties at aspect ratios of 2:1 or higher. It has also been demonstrated that silicon nitride films deposited according to this embodiment under high temperature processing conditions (above 580xc2x0 C.) can have a WER of 15 xc3x85/min or less.
In part, the excellent physical characteristics of these silicon nitride films are achieved because the films can be deposited at pressure and RF power levels that were previously not possible in other chambers. For example, in a more preferred embodiment, the ratio of LF power to total RF power is greater than 50%, while in another preferred embodiment, the silicon nitride deposition sequence takes place at a pressure between 2 and 5 torr. The physical characteristics are also achieved in part through the use of the conical holes, which in turn increases the plasma density and ionization efficiency of the created plasma thus allowing an increased amount of N2 as compared with NH3 to be used in the film""s process gas. Reduced NH3 content in the process gas results in less hydrogen in the film and a lower WER.
In another embodiment, a bipolar low frequency asymmetric RF waveform, also referred to as a triangular or sawtooth waveform, is employed to control ion bombardment. Such an asymmetric RF waveform enhances ion bombardment at the substrate while hindering the formation of harmonics, which the present inventors have discovered can provoke plasma sheath instabilities.
In another embodiment of the present invention, a substrate processing system includes a deposition chamber having a reaction zone, a plasma power source for forming a plasma within the reaction zone of the deposition chamber, and an impedance tuning system. The plasma has a first impedance level that can be adjusted by the impedance tuning system to a second impedance level. Such an adjustment acts as an additional xe2x80x9cknob of controlxe2x80x9d providing another method for process engineers to use to change and tune the properties of films deposited within the reaction zone. In a preferred version of this embodiment, the impedance tuning system includes a variable capacitor.
In still another embodiment of the present invention, a substrate processing system includes a deposition chamber with a reaction zone, a substrate holder for holding a substrate in the reaction zone during substrate processing, a gas distribution system for supplying a process gas to the reaction zone, a plasma power source for forming, within the reaction zone, a plasma from the process gas and an impedance monitor that is electrically coupled to the deposition chamber and that can measure the impedance level of said plasma. The substrate processing system can also include a computer processor that receives the measured impedance level as an input. The processor can be connected to various systems of the substrate processing chamber, such as the gas distribution system, a pressure control system and/or the RF generator and adjust processing conditions according to the measured impedance level. Such an adjustment can be made, for example, near the end of an extended wafer run (e.g., a 2000 wafer run) where the measured impedance of the chamber may change during the course of the run. In this example, the processor could adjust processing conditions if, or when, the impedance level of the chamber drifted outside of a predefined range. The adjustment can include adjusting the chamber pressure, temperature, plasma power level (e.g., RF power level) or a similar process variable. Also, if the substrate processing system included a impedance tuning system, the adjustment could include directly adjusting the impedance of the chamber with that system.
These and other embodiments of the present invention, as well as its advantages and features are described in more detail in conjunction with the text below and attached figures.